The invention relates generally to combustors, and more particularly to a coanda injection system for axially staged low emission combustion devices.
A gas turbine employed in a gas turbine plant or a combined cycle plant is operated to achieve higher operational efficiency under higher temperature and higher pressure conditions, and this tends to increase emissions (for example, NOx) in an exhaust gas stream. Although various factors for generation of NOx are known, the dominant one is flame temperature in a combustor. NOx emissions are directly proportional to the flame temperature in a combustor.
There are some conventional techniques for reducing NOx in an exhaust gas stream from a combustor. One conventionally adopted method involves injection of steam or water into the high-temperature combustion area in a combustor for reducing the flame temperature during the combustion. Although this method is easy to perform, it suffers from problems in that a large amount of steam or water is required, resulting in reduced plant efficiency. Additionally, injection of a large amount of steam or water into a combustor increases combustion vibrations, partial combustion products, and reduces life.
Taking the above defects into consideration, a dry type premixed lean combustion method has been developed, in which fuel and combustion air are injected in a premixed mode and burned under lean fuel conditions in a single stage combustor. Even though reduction of NOx emissions is achieved, the operability range of the combustor is reduced due to the premixed injection mode. The usage of a single stage combustion in a combustor may not guarantee lower NOx emissions.
Multi-stage combustion may be used to achieve reduced NOx emissions and better operability range of a combustor. In such conventional systems, the additional premixers are provided in an environment of the later stages of the combustor having reacting gas flows from one or more primary nozzles. The presence of premixers disturbs the flow pattern of hot gases in the later stages of the combustor resulting in higher pressure drops across the combustor. Cooling of such premixers is also difficult due to elevated temperatures and the introduction of flammable mixtures in later stages of combustors.
Accordingly there is a need for a system that is employed in gas turbines that achieves reduced NOx emissions from the axially staged combustor without compromising the dynamics and operability of the combustor.